MDEV-38558 : MariaDB does not support writes during MySQL --> MariaDB migration

When MariaDB node has joined to MySQL Galera cluster if any writes
to existing tables are done cluster requests to use protocol
version 7 (max protocol version used by MySQL Galera cluster).
MariaDB had support to protocol version up to 4. While writeset
keys are prepared allow using protocol version 7, it has no
effect on writeset key format i.e. protocol version 4 and 7
are identical on that case.

Note that there is no mtr test case because currently there is no
way to test both MySQL and MariaDB in mtr.

Author: janlindstrom

sql/wsrep_mysqld.cc

@@ -1,5 +1,5 @@
 /* Copyright (c) 2008, 2025, Codership Oy <http://www.codership.com>
-   Copyright (c) 2020, 2025, MariaDB
+   Copyright (c) 2020, 2026, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -121,6 +121,10 @@ my_bool wsrep_restart_slave_activated= 0;       // Node has dropped, and slave
 bool wsrep_new_cluster= false;                  // Bootstrap the cluster?
 int wsrep_slave_count_change= 0;                // No. of appliers to stop/start
 int wsrep_to_isolation= 0;                      // No. of active TO isolation threads
+//
+// NOTE : MySQL Cluster has max_protocol version 7
+// thus protocol versions 5-7 are reserved for compatibility.
+// 
 long wsrep_max_protocol_version= 4;             // Maximum protocol version to use
 long int  wsrep_protocol_version= wsrep_max_protocol_version;
 ulong wsrep_trx_fragment_unit= WSREP_FRAG_BYTES;
@@ -1973,6 +1977,10 @@ static bool wsrep_prepare_key_for_isolation(const char* db,
   case 2:
   case 3:
   case 4:
+  // For MySQL Galera Cluster migration
+  case 5:
+  case 6:
+  case 7:
   {
     *key_len= 0;
     if (db)
@@ -1990,7 +1998,6 @@ static bool wsrep_prepare_key_for_isolation(const char* db,
     break;
   }
   default:
-    assert(0);
     WSREP_ERROR("Unsupported protocol version: %ld", wsrep_protocol_version);
     unireg_abort(1);
     return false;
@@ -2153,6 +2160,10 @@ bool wsrep_prepare_key(const uchar* cache_key, size_t cache_key_len,
     case 2:
     case 3:
     case 4:
+    // For MySQL Galera Cluster migration
+    case 5:
+    case 6:
+    case 7:
     {
         key[0].ptr= cache_key;
         key[0].len= strlen( (char*)cache_key );
@@ -2164,6 +2175,7 @@ bool wsrep_prepare_key(const uchar* cache_key, size_t cache_key_len,
         break;
     }
     default:
+        WSREP_ERROR("Unsupported protocol version : %ld", wsrep_protocol_version);
         return false;
     }